The two most abundant and renewable biomaterials in nature, cellulose and chitin, are chemically similar, semi-crystalline, and both form natural hierarchical composites. Similar to the role of cellulose in trees and plants, chitin serves as a major component in the supporting tissues of crustaceans, fungi, mushrooms, insects, etc. Annual availability of chitin is about 1 to 100 billion ton (Nair 2003) with shrimp and crab wastes as the principal source of raw materials. The United Nations Food and Agricultural Organization (FAO) estimated that the global production of shrimp and crab from 1995 to 1999 was 4,021,521 and 1,299,464 ton/y, respectively, and the fisheries normally produce a crustacean waste of about 30% or about 1.6×106 ton/y. Although 150,000 ton/y of chitin are currently available for commercial/industrial applications, only a few thousand tons are actually used for worldwide commercial applications. As a natural substance in various foodstuffs, chitin is considered as a safe material. To date, chitin and its derivatives have shown promise in the biomedical, food, cosmetic, and textile industries due to their biocompatibility, biodegradability, antimicrobial properties, and high tensile strength. Thus, it is important to further develop efficient and widespread use of chitin as a natural and eco-friendly material.
Chitin is a linear polysaccharide, white and porous material; consisting of β-(1→4)-2-deoxy-2-acetamido-D-glucopyranose, i.e., the structure of chitin is similar to cellulose, except that the C2-hydroxyl group of cellulose is replaced by an acetamide group in chitin. The chitin molecules exhibit helicoidal and microfibrillar structures comprising nanofibers about 2-5 nm in diameter and about 300 nm in length embedded in several protein matrices (Raabe 2006). Chitin has three crystalline polymorphs, α, β and γ, which are organized as sheets of chitin chains held tightly by a number of strong C—O . . . H—N interchain hydrogen bonds. α-Chitin, the most abundant form, contains alternating antiparallel chains per unit cell whereas β-chitin contains only parallel chains per unit cell. γ-Chitin has two chains running in one direction and another chain running in the opposite direction and its X-ray diffraction is very similar to that of α-chitin (Jang 2004). Therefore, the crystalline nature of α-chitin inhibits its solubility in most organic solvents whereas β-chitin is more susceptible to swelling, especially in aqueous media. This could be the reason why α-chitin has limited applications despite its available abundance. Nevertheless, deacetylation of chitin leads to water-soluble chitosan, which has been more widely studied and explored for various applications (Azzaroni 2005).
Analogous to cellulose, chitin comprises both crystalline and amorphous domains and the latter can be hydrolyzed to release the crystalline segments. Cellulose nanocrystals (CNC) have received significant attention as biomaterials for diversified applications and several production methods have been established for this nanoscale material (Lam 2012; Leung 2012; Leung 2013). In contrast, there are only a few reports on the synthesis of chitin nanocrystals (ChNCs) by subjecting chitin to acid hydrolysis and mechanical disruption (Goodrich 2007; Fan 2008). Acid hydrolysis in these reports requires an expensive chemical, TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) to prepare oxidized chitin nanocrystals or oxidized cellulose nanocrystals obtained from acid hydrolysis. Acid hydrolysis randomly cleaves the non-crystalline regions of the microfibrils to produce stable colloidal suspensions of rod-like particles of nanoscale dimensions (Revol 1993; Li 1997). As with CNCs, α-chitin nanocrystals have several advantageous properties, such as small size and high particle aspect ratio, high surface area, high stiffness and strength, wide availability and renewability, low density, and ease of chemical modification. Indeed, chitin may offer more design versatility over cellulose, considering the presence of both hydroxyl and amine/N-acetyl functionalities on the surface for chemical modification and conjugation.
RU 2256601 (Chvalun 2005) discloses the use of a radical initiator (e.g. potassium persulfate) for polymerization of acrylamide to polyacrylamide in the formation of nanocomposites containing chitin nanocrystals. Persulfate is only present in low concentrations of (0.1-1.0 wt %) with the reaction performed at basic pH, which is insufficient to achieve oxidation and hydrolysis of chitin. Nanocrystalline chitin is created by mechanical disintegration. JP-S59113185 (Tomoji 1984) discloses the treatment of chitin-based waste with an oxidant, one of which could be sodium persulfate, to deodorize chitin-based industrial waste. The resulting chitin is further treated with various chemicals to remove dust from used metal. Persulfate is not used for the effective hydrolysis of chitin at acidic pH and oxidation of chitin to form highly crystalline nano-chitin. U.S. Pat. No. 4,931,551 (Albisetti 1990) discloses the use of an oxidizing agent for breaking down, bleaching and dispersing chitin in an alkaline aqueous medium. The main purpose of the oxidant is the dispersion of chitin as the size of chitin remains unchanged during the course of treatment.
There remains a need for a simple effective method for producing chitin nanocrystals.